Normally, ultrasound flow meters suited for measuring a fluid flow in connection with charging of a consumed quantity (e.g. heat, cooling, water or gas) will have a housing, which can be metallic or polymeric, with a cavity in the form of a through-going hole for receiving a fluid flow to be measured. Connection means to other fluid flow elements are present in each of the housing ends.
In the housing a number of ultrasound transducers are installed for measuring the velocity of the fluid flow. In most flow meters two ultrasound transducers are used for sending, respectively receiving, an ultrasound signal, but versions with one ultrasound transducer, as well as versions with more than two ultrasound transducers, are seen. Furthermore, an electronic circuit for operation of the ultrasound transducers typically mounted in a separate enclosure and fastened to the housing. Most often the electronic circuit is implemented on a Printed Circuit Board (PCB).
In typical metallic flow meter housings, through-going holes are present between the exterior of the housing and the measuring tube, enabling ultrasonic signals to penetrate from the exterior of the housing, through the holes, to the measuring tube. The through-going holes are necessary since the ultrasonic signals are poor at penetrating directly through metallic materials. In order to protect the ultrasound transducers from the effects of contact with the fluid flowing through the housing, a protection membrane is inserted between the fluid and the ultrasound transducer. More specifically, each of the ultrasound transducers is glued onto a thin, sheet metal membrane then put into an injection moulded polymeric part, which also contains the electrical connection to the ultrasound transducer. This subassembly is then placed in the flow meter housing.
However, one common problem with prior art ultrasound flow meters is that the level of complexity, and consequently high expenditures and large timeframes, involved with the manufacturing are too high.
A further problem is that the mentioned way of manufacturing an ultrasound flow meter necessitates that several parts are dedicated for used with one specific type of housing, and thus a large number of specially designed parts must be manufactured for each flow meter version.
Still further, it is a problem that it is difficult to protect electronic parts of a flow meter against humidity such that these parts can have a long life-time, e.g. of 20 years, even if the meter is installed in a humid environment.
In WO 2008/129050 a device for determining and/or monitoring the volume and/or mass flow of a medium flowing through a conduit is disclosed. In this disclosure two ultrasound sensors are mounted on a base plate, and at least one of the ultrasound sensors is movable relative to the second ultrasound sensor. Fastening means are provided via which the device can be clamped on a fluid pipe. The device is a so-called clamp-on sensor where the ultrasonic signals are coupled in to the pipe through the base-plate and through the pipe wall.
In DE 920 29 70U a device for measuring a fluid stream in a pipe line is disclosed. Ultrasonic transducers are placed inside a casing enveloping the pipe line. Ultrasonic signals are coupled in to the pipe line through the pipe wall
In EP 0 945 712 A discloses a device for measuring a gas flow, the device comprises ultrasonic transmitting and receiving units which are aligned against openings of a measuring chamber. Diaphragms are arranged between the measuring chamber and the transmitter and receiver unit. The diaphragms are permeable for ultrasonic waves, but substantially impermeable for moisture and bacteria. Each transducer has a separate diaphragm and is not positioned within a common casing.